Thermal hydrolysis, anaerobic digestion and dewatering of sewage sludge as a best first step in sludge strategy: full scale examples in large projects in the UK and strategic study including cost and carbon footprint.

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Courtesy of Cambi AS

Abstract

This paper shows that by using thermal hydrolysis process (THP) as a pre-treatment for anaerobic digestion (AD) it is possible to develop a step wise approach to sludge strategy that is cost effective and flexible. By generating a low volume of non odorous, well dewatered and safe digested cake it is possible to recycle sludge directly to agriculture or dry it for fuel use or incinerate/gasify it. In every case the financial cost, carbon footprint and energy balance is better than carrying out the same final processes on undigested or conventionally digested sludges.

The thermal hydrolysis process was first put into practice in Norway in 1995 and is now in application in 20 projects, around the world, for 12 million population equivalent. In simple terms the process relies on pressure cooking sludge cake at 165C for 30 minutes before feeding the liquefied sludge at about 10% DS to conventional digesters. It has the benefits of giving very and rapid digestion, high bioconversion rates – even for secondary sludge and also leads to extremely good dewatering properties – this has been discussed in a number of papers at IWA conferences over the years and can be studied at the following web page (www.cambi.com )

The results of a study carried out by Entec UK Ltd for Thames Water Utilities in 2007/8 are discussed. This study was a thorough review of the range of technologies available for treatment of sludge for London and the surrounding area. As such it is relevant for large population centres embarking on new sludge treatment programmes.

The various process/disposal routes were ranked in league tables in terms of whole life costs, energy consumption and greenhouse (GHG) gas emissions.

Numerous tables are included for the individual process/disposal routes and the league tables.

In general process/disposal routes including AD and THP had the lowest whole life cost, energy consumption and GHG emissions. Entec’s study shows that advanced digestion with agricultural use was best option for the areas outside of London. Within Metropolitan London then a focus on extreme volume reduction or thermal destruction following THP and AD was more important. Thames Water Utilities has now published its draft 5 year business plan that includes THP plants for the two major London WWTPs as a way of supplementing it’s incinerator capacity.

Full scale examples are given for 3 projects (Aberdeen, Scotland; Dublin, Ireland and Manchester, England) that have thermal hydrolysis, digestion and dewatering as a first step for projects varying between 0.5 and 3 million population equivalents.

The example for Aberdeen (0.5 million P.E.) is for THP/AD and dewatering that produces a pasteurised cake at 34% dry solids. The plant also produces 1.5 MW of electricity. The biosolids product as proved popular for local farmers and demand exceeds supply as there is only 20,000 tonnes of final product a year. A mass and energy balance is given for this plant that has been in operation 8 years.

In Dublin (1.7 million P.E.) THP and AD is coupled with thermal drying. Compared to conventional digestion the requirement for drying is half because the high solids destruction and high dewaterability halve the water evaporation requirements. A comparison of dryer evaporative capacity is shown in the paper plus an energy balance. The digestion plant is very compact consisting of 3 digesters at 4250m3 each and was built in 2000.

The Manchester project, planned for 2009/11 will make use of an existing digestion plant at Daveyhume where there are 8 digesters at 7,500m3 each. The existing sludge from this plant (1.2 million P.E.) is sent to a nearby incinerator – however the sludge does not dewater well and supplementary fuel is required. United Utilities originally planned to build a new incinerator north of Manchester to incinerate sludge from a number of plants – mainly as raw sludge. However a study by their engineering team showed the best option was to retrofit THP at Daveyhume and expand the treatment capacity to about 3 million P.E. by increasing the solids loading to the digesters. The high solids sludge after dewatering will be close to autothermic increasing the effective sludge capacity of the existing incinerator and any remaining cake will be taken to agriculture. In addition electricity production will increase from 4 MWs to 10 MWs. The upgrade is in the planning/construction phase.

The study and review of these projects demonstrates the flexibility of projects that in THP and AD in combination as the reduced volume of high quality cake makes decisions in sludge use of destruction easier. Furthermore the high solids loading of these digesters (2 -3 times higher than conventional) makes the THP self financing in many cases compared to new digester construction.

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